However there are a few clues.
First, night pain that awakens you from sleep; second the knee
hurts when you bend
it, and gets stiff if you sit with it bent; finally, buckling when you stand. These three combined, indicate a mechanical problem such as a torn cartilage possibly combined with a fluid collection, known as an effusion, in the knee
You mentioned no trauma, but didn't say whether you are active or not.